Process for yttriding and rare earthiding



United States Patent US. Cl. 29-194 11 Claims ABSTRACT OF THE DISCLOSUREA metallide coating of yttrium or rare earth metal is formed on certainspecified base metals by forming an electric cell containing said basemetal as a cathode and a carbon anode or the coating metal as an anodeusing a specified fused salt electrolyte maintained at a temperature ofat least 500 C., but below the melting point of said metal compositionand controlling the current flowing in said electric cell, so that thecurrent density of the cathode does not exceed 30 amperes/dmF. Thedeposited metal diffuses into the substrate so that the metallidedcoating is the composition containing both the deposited metal and themetal of the substrate. This process is therefore useful in forming adiffusion coating of the deposited metal and the substrate metal on thesurface of the substrate.

This invention relates to a method for metalliding a base metalcomposition. More particularly this invention is concerned with aprocess for electrolytically yttriding and rare earthiding a base metalcomposition in a fused salt bath, containing a fluoride of the metal tobe deposited. The process can either be operated as a battery generatingits own electromotive force or as an electrolysis cell wherein a currentis supplied from an Outside direct current source.

I have discovered that a uniform tough, adherent yttride and rareearthide coatings can be formed on a specific group of metals and alloysemploying current densities in the range of 0.10 to 30 amperes/dm.

In accordance with one aspect of the process of this invention, theyttrium or rare earth metal is employed as the anode and is immersed ina fused salt bath composed essentially of a member of the classconsisting of the alkali metal fluorides, mixtures thereof, and mixturesof the alkali metal fluorides with strontium and barium fluorides andcontaining from 0.-0140 mole percent of yttrium or a rare earth metalfluoride. The cathode employed is the base metal upon which deposit isto be made. I have found that such a combination is an electric cell inwhich an electric current is generated when an electrical connection,which is external to the fused bath, is made between the base metalcathode and the metal anode. Under such conditions, the anode metaldissolves in the fused salt bath and anode metal ions are discharged atthe surface of the base metal cathode where they form a deposit of theanode metal which immediately diffuses into and reacts with the basemetal to form a metallide coating. In the specification and claims I usethe term yttriding, rare earthiding" and metalliding to designate anysolid solution or alloy of yttrium or a rare earth metal and the basemetal regardlessof whether the base metal does or does not form anintermetallic compound with yttrium or the rare earth metal in definitestoichiometric proportions which can be represented by a chemicalformula.

The rate of dissolution and deposition of the yttrium or rare earth isself regulating in that the rate of deposition is equal to the rate ofdiffusion of the yttrium or rare earth into the base metal cathode. Thedeposition rate can be decreased by inserting some resistance in thecircuit. A faster rate can be obtained by impressing a limited amount ofvoltage into the circuit to supply additional direct current.

The amount of the yttrium or rare earth metal fluoride present in thebath can be from 0.01 to 40 or more mole percent. It is preferredhowever that the concentration of the yttrium or rare earth fluoride befrom 0.1 to 10 mole percent of the fused salt bath. Higherconcentrations of the yttrium or rare earth fluorides, i.e., 30 molepercent or more, are necessary only Where in the particnlar instance thedisplacement of lithium ions by yttrium or rare earth metal is to bealmost completely suppressed.

The alkali metal fluorides which can be used in accordance with theprocess of the invention include the fluorides of lithium, sodium,potassium, rubidium and cesium and mixtures thereof. However, it ispreferred to employ an eutectic mixture of sodium fluoride and lithiumfluoride because some free alkali metal is produced by a displacementreaction and potassium, rubidium and cesium are volatilized with theobvious disadvantages. It is particularly preferred to employ lithiumfluoride as the fused salt bath in which the yttrium or rare earthfluoride is dissolved, because at the temperatures at which the cell isoperated, lithium metal is not volatilized to any appreciable extent.Mixtures of the alkali metal fluorides with strontium fluoride or bariumfluoride can also be employed as a fused salt in the process of thisinvention.

The chemical composition of the fused salt bath is critical if goodmetallide coatings are to be obtained. The starting salt should be asanhydrous and as free of all impurities as is possible or should beeasily dried or purified by simply heating during the fusion step. Theprocess must be carried out in the substantial absence of oxygen sinceoxygen interferes with the process by forming anode metal oxides andthereby preventing a firmly adhering film of the anode metal from beingdeposited on the base metal cathode. Thus, for example, the process canbe carried out in an inert gas atmosphere or in a vacuum. By the termsubstantial absence of oxygen it is meant that neither atmosphericoxygen nor oxides of metals are present in the fused salt bath. The bestresults are obtained by starting with reagent grade salts and bycarrying out the process under vacuum or an inert gas atmosphere, forexample, in an atmosphere of argon, helium, neon, krypton or xenon.

I have sometimes found that even commercially available reagent gradesalts must be purified further in order to operate satisfactorily in myprocess. This purification can be readily done by utilizing scrap metalarticles as the cathodes and carrying out the initial metalliding runswith or without an additional applied Voltage, thereby platingoutandremov ng.fr mth bath those impurities which.

interfere with the formation of high quality metallide coatings.

The base metals which can be metallided in accordance with the processof this invention included the metals having atomic numbers of 4, 21,22, 25 to 29, 40, 43 to 47, 72 and '75 to 79, inclusive. These metalsare, for example, beryllium, scandium, titanium, manganese, iron,cobalt, nickel, copper, zirconium, technetium, ruthenium, rhodium,palladium, silver, hafnium, rhenium, osmium, iridium, platinum and gold.Alloys of these metals with each other or alloys containing these metalsas the major constituent, that is, over 50 mole percent, alloyed withother metals as a minor constituent, that is, less than 0 mole percent,can also be metallided in accordance with my process, providing themelting point of the resulting alloy is not lower than the temperatureat which the fused salt bath is being operated. It is preferred that thealloy contain at least 75 mole percent of the metal and even morepreferred, that the alloy contain 90 mole percent of the metal withcorrespondingly less of the alloying constituent.

Although it has been found that it is not possible to metallide thehighly refractory metals such as vanadium, chromium, niobium,molybdenum, tantalum and tungsten, I have unexpectedly discovered thatif such refractory metals are covered with a thin layer of one ofseveral specific iding agents, that these metals can then besatisfactorily yttrided or rare earthided. The special iding agentswhich I have found useful are beryllium, boron and silicon. These alloysare hereinafter referred to as beryllides, borides and silicides,respectively. The beryllided compositions are produced in accordancewith the process of United States Patent No. 3,024,175, the silici dedcompositions in accordance with United States Reissue Patent No. 25,630,the borided compositions in accordance with the process of United StatesPatent 3,024,176, which patents are made a part hereof.

I have also found that it is advantageous to conduct the metallidingprocess in the absence of carbon, because carbon forms a very stablemetal carbide on the surface of the base metals thereby rendering itvery difficult to further metallide the base metal and giving lessfirmly adhering deposits. I have found that carbon can be removed fromthe fused salt bath by operating it as a cell, until the carbide coatingis no longer formed on the surface of the base metal.

The form of the anode is not critical. For example, I can employ as theanode pure yttrium or rare earth metal in the form of a rod or theyttrium or rare earth metal can be employed in the form of chips inporous metal baskets, such as niobium or tantalum. I have also foundthat a shielded carbon anode can be substituted for the metal anode andthe cell operated as an electrolytic cell by impressing someelectromotive force from an outside source as hereinafter described.

In order to produce reasonablyfast plating rates and to insure thefusion of the yttrium or rare earth into the base metal to form ametallide, I have found it desirable to .operate my process at atemperature in the range of from about 500 C. to 1100 C. It is preferredto operate at temperatures of from 9001100 C.

The temperature at which the process of this invention is conducted isdependent to some extent upon the particular fused salt bath employed.Thus, for example, when temperatures as low as 500 C. are desired, aeutectic of sodium, potassium and lithium fluoride can be employed.Inasmuch as the preferred operating range is from 900 C. to 1100 C., Iprefer to employ lithium fluoride as the fused salt.

When an electrical circuit is formed external to the fused salt bath byjoining the metal anode to the base cathode by means of a conductor, anelectric current will flow through the circuit without any appliedelectromotive force. The metal anode acts by dissolving in the fusedsalt bath to produce electrons and anode metal ions. The electrons flowthrough the external circuit formed by the conductor and the anode metalions migrate through the fused salt bath to the base metal cathode to bemetallided, where the electrons discharge the ions forming a metallidecoating. The amount .of current can be measured with an ammeter whichenables one to readily calculate the amount of metal being deposited onthe base metal cathode and being converted to the metallide layer.Knowing the area of the article being plated, it is possible tocalculate the thickness .of the metallide coating formed, therebypermitting accurate control of the process to obtain any desiredthickness of the metallide layer.

Although the process operates very satisfactorily as a battery withoutimpressing any additional electromotive force on the electrical circuit,I have found it possible to apply a small voltage when it is desired toobtain constant current densities vduring the reaction, and to increasethe deposition rate of the metal being deposited without exceeding thediffusion rate of the metal into the base metal cathode. The additionalshould not exceed 1.0 volt and preferably should fall between 0.1 and0.5 volt.

Since the diffusion rate of yttrium and rare earth metals into thecathode article varies from one material to another, with temperature,and with the thickness of the coating being formed, there is always avariation in the upper limits of the current densities that may beemployed. Therefore, the deposition rate of the. iding agent must alwaysbe adjusted so as not to exceed the diffusion rate of the iding agentinto the substrate material if high efficiency and high qualitydiffusion coatings are to be obtained. The maximum current density forgood metalliding is 30 amperes/dm. when operating within the preferredtemperature ranges of this disclosure. Higher current densities cansometimes be used to form coatings with yttrium or the rare earth metalsbut in addition to the formation of a metallide coating, plating of theiding agent occurs over the diffusion layer.

Very low current densities (0.010.1 ampere/dm. employed when diffusionrates are correspondingly low, and when very dilute surface solutions orvery thin coatings are desired, can only be used profitably when highconcentrations of the iding ion 5%) or lithium metals (-1%) are in thesalt. Often the compositions of the diffusion coating can be changed byvarying the current density, producing under one condition a compositionsuitable for one application and under another condition a compositionsuitable for another application. Generally, however, current densitiesto form good quality metallide coatings fall between 1 and 10 amperesper dm. for the preferred temperature ranges of this disclosure. Thehigher current densities (10-30 an1ps/dm. can only be used on a fewmetals, such as nickel and cobalt, Where diffusion is very rapid due totremendous reactivity in the formation of intermetallic compounds.

If an applied is used, the source, for example, a battery or othersource of direct current, should be connected in series with theexternal circuit so that the negative terminal is connected to theexternal circuit, terminating at the base metal being metallided and thepositive terminal is connected to the external circuit terminating atthe metal anode. In this way, the voltages of both sources arealgebraically additive.

As will be readily apparent to those skilled in the art, measuringinstruments such as voltmeters, ammeters, resistances, timers, etc., maybe included in the external circuit to aid in the control of theprocess.

I have found that when a base metal has been metallided in accordancewith the process of this invention, that such metallided base metalshould be removed from the fused salt electrolyte within a reasonabletime. It has been found that if such metallided base metal compositionis allowed to remain in contact with the fused salt bath, that adisplacement reaction takes place wherein the yttrium or rare earth inthe metallided base metal reacts with the alkali metal fluoride toproduce free alkali metal and the fluoride of the yttrium or the rareearth metal. 5

The description of the process of this invention as given hereinabovehas been primarily described with reference to the employment of ayttrium or rare earth metal anode and operating the process as abattery. I have also found that a shielded carbon anode can be used inplace of the yttrium or rare earth metal anode and the cell operated asan electrolysis cell wherein the electromotive force which allOWs thecell to operate is derived from an outside source of direct current suchas a battery.

I have found that in the electrodeposition and metalliding with yttrium,or the rare earth fluorides, that the limitations on current density asset forth above should be maintained. It has been found however that thevoltage applied to the electrolytic cell when a carbon anode is employedmust be increased to from about 1 to 3.0 volts. If the carbon anode hasinsufficient surface area, voltage can, of course, go much higher.

When the cell is operated with a carbon anode, carbon tetrafluoride isproduced at the anode and escapes as a gas, and the metal ions migrateto the cathode where they are deposited and diffuse into and react withthe base metal to form a metallide coating. Extended operations withcarbon anodes would obviously require the addition of yttrium or rareearth fluorides from time-to-time as the iding ions were depleted fromthe salt.

The following examples serve to further illustrate my invention. Allparts are by weight unless otherwise stated.

Example 1 Lithium fluoride (6810 grams) was charged into a Monel liner(5%" diameter x 12" deep) fitted into a mild steel pot (6" in diameter x18" deep). The pot was placed in an electric furnace (6 /2" in diameterx 20" deep). The mild steel pot was flanged at the upper portion andsealed with a cover plate of nickel plated steel which contained a waterchannel for cooling, two ports (2%" in diameter) for glass electrodetowers and two 1" ports for a thermocouple probe and a gas bubbler orvacuum connection. A vacuum was pulled on the cell and the lithiumfluoride melted. Argon was then bled into the cell and yttrium fluoride(91 grams) was added to the molten lithium fluoride. A 4" diametergraphite anode shielded with a Monel screen to prevent carbon particlesfrom getting into the salt, Was then immersed in the molten lithiumfluoride to a depth of 4 inches. A nickel strip (1" x 5" x 0.02") wasemployed as a cathode and the temperature maintained at 1000 C. for thetimes, volts and amperes as set forth in the following table.

TABLE I Volts (anode polarity) The nickel cathode had gained 0.014 gramin weight, (theoretical yield 0.270 gram, for cathode reaction of Y++3ey) and was dull grey in color. An X-ray emission analysis showed thepresence of large amounts of yttrium on the surface of the nickel.

results at low current densities.

TABLE II Volts (anode polarity) Current density, amps/rim.

0 8 Current on. 3 Current 01f. 0 Sample out.

The nickel cathode gained 0.129 gram of yttrium, which corresponds to a60% yield, and developed a yttride coating 1 mil thick. The batterycharacteristics of the reaction are obvious from the negative polarityof the anode.

Table III gives the conditions of a high current density run at 1000 C.employing a nickel cathode and yttrium anode.

TABLE III Volts (anode Current density, T1me (mm.) polarity) amps/dmfl-O. 420 0 +0. 320 10 Current on. +0. 56 10 Current ofi. 0. 16 0 -0. 20 0Sample out.

The nickel cathode gained 0.266 gram of yttrium, which corresponds to a91% yield, and developed a 2 mil thick coating.

It was found that the yttrided samples should be removed from the cellimmediately after removal of the current, because allowing the yttridedsample to remain in contact with the lithium fluoride bath permitted theyttrium to go into solution displacing lithium ions. This wasdemonstrated by placing two samples of yttrided nickel in a bath fordifferent lengths of time. The first for 60 minutes and the second for10 minutes respectively. The sample remaining in the bath for 60 minuteslost 0.062 gram Whereas the sample remaining in the bath for 10 minuteslost only 0.020 gram. A nickel strip placed in the bath was unafiected.

Example 3 In a series of runs at 850 to 1100 C. using nickel strips ascathodes and shielded carbon rods as anodes, current density and yttriumfluoride concentration were found to have large effects on the yield ofyttriding, as shown in Table IV.

TABLE IV Average YF eoulombic concentration, Volts, anode Currentdensity, etficiency mole percent polarity amps/dm. (percent) In all theruns shown in Table IV, shiny, smooth yttride coatings were produced onthe nickel cathodes at all the Example 4 A large number of runsemploying base metal cathodes were made in a lithium fluoride cell asshown in the following table, which gives the conditions of the reactiontogether with the results.

TABLE V Current Weight Percent Temp., Time Density, gain, coulombic(min.) amps/(1111. grams efliciency Description of coating 1,050 2 30.00.705 96 mil coat; shiny, smooth, flexible, hard (600- QOOIKHBfIJ). 9002 16.7 0.140 95.5 1 mi coa 900 16 2.1 0.105 71.5 0.7 mil coatfig gfi iflexlblei hard (500 900 32 1 0.058 39 13 (1L4m1il coa: 900 2 23.6 0.158mi coa Smooth fl h d 900 3 9.4 0.147 67 1 mil co t} 1 Y, very an a Do.900 1.2 0.009 5 0.1 mil coat (500-800 KEN) Ir0n-chromium(28) 900 8 5.10.146 1 mil coat; shiny, grainy, flexible, soft, mainly plate plus somediffusion layer. Zirconium 900 2 14.3 0.042 57 0.2 mil coat;shiny,smooth, very flexible. Rhenium 900 3 1.3 0.085 17 -01 mil coat; shiny,smooth, flexible, surface softer than rhenium, some diffusion, mostlyplating. Platinum 900 8. 5 1. 3 0. 095 100 2 mil coat; shiny, smooth,brittle, hard. Palladium. 900 12 3.3 0.018 100 1 Islgat; shiny, smooth,brittle, hard (-500 Silicided molybdenum* 900 1 32 0.027 36 0.5 milcoat; shiny, smooth, fairly flexible, soft plated yttrium surface overhard ternary layer *The molybdenum had been coated with a silicidecoating in accordance with Reissued Patent No. 25,630, reissued Aug. 4,1964.

Example 5 A carbon electrode was substituted for the yttrium anode inthe previous example and some additional metals yttrided in accordancewith the same general procedure with the results given in Table VI.

The cell was then run for 5 ampere hours against 7 different nickelcathodes to remove impurities from the cell. At the end of these runs,the nickel cathode strips came out shiny and smooth and the yield hadincreased to 70% of theoretical.

TABLE VI Current Percent Temp., Time density, Volts, anode Weight gain,coulombic Metal 0. (min.) arnps/dm. polarity grams efficiencyDescription of coating Borided niobium 940 3 26 +3-5 0.105 10 1 milY-B-Nb coat; shiny smooth,

very flexible, soft plated yttrium surface over hard inner ternarylayer. Borided (304) stain- 940 3 l5 +2-3 0.153 56 1 mil Y-B-SS coat;shiny, bumpy less steel. (from surface melting) moderately flexible,soft yttrium surface over hard boride inner layer. Stainless steel 98010 8 +2-4 0. 480 31 1 mil coat; shiny, bumpy (from surface melting),moderately flexible, hard (700-800 KHN). Monel 1,100 10 8 +2-3 0.165 1.2mil coat; shiny, smooth, moderately hard (400-500 KHN). Borided nickel940 3 25 +2-3 0. 462 84 -3 mil Y-B-N i coat; 3 mil B-Ni coat;

smooth, shiny, very hard, flexible.

Example 6 Example 7 the salt through the other port as a cathode. Thecell was maintained at 945 C. and was run as indicated in the followingtable.

TABLE VII Volts, anode polarity Current density, amps/(1m.

Current on.

Current off.

The nickel sample was covered with a black material which readily washedoff, revealing a hard grey, flexible coating which was 2 mils thick. Thenickel cathode sample had gained 0.835 gram as compared to a theoreticalof 1.955 (for the reaction Gd+ +3e Gd) for a 43% yield. X-ray emissionspectra showed the presence of large amounts of gadolinium in thesurface of the nickel strip.

A gadolinium anode was substituted for the carbon anode of Example 6 andthe cell run employing a nickel strip as a cathode (4" x 1" x 0.020") ata temperature of 1000 C. in accordance with conditions set forth in thefollowing table.

TABLE VIII Volts, anode Current density, polarity amps/dm.

+1.03 10. 0 Current on. +1.02 10.0 Current ofi. -0. 16 0 Sample out.

The nickel cathode had gained 0.628 gram of a theoretical 0.652 gram anddeveloped a coat that was shiny, smooth, hard and flexible and was 2mils thick.

When the cell was operated employing a nickel cathode at lower currentdensities (1-3 amps./dm. yields of 40-50% were obtained.

The cell was then run employing the procedure of Example 5 with agadolinium or carbon anode and a metal cathode under the conditions andwith the results shown in the following table.

TABLE IX Current Percent Temp., Time, Volts, anode density, Weight gain,coulomblc Metal 0. min. polarity amps/dm. grams efliciency Descriptionof coating 1015 steel l, 000 +0. 42 5 0. 159 40 0.5 mil coat; shiny,smooth, flexible,

moderately hard.

Cobalt 1, 000 4 +0. 75 0. 542 83 2mil coat; shiny, smooth,flexible,hard.

Vanadium 1 1, 000 120 -0. 11-0. 60 8 0. 064 8 0.3h ml coat; shiny,smooth, flexible,

Copper 900 2 +1. 0 5 0. 060 90 Surface melted, shiny soft, flexible.

Palladium 900 3 +0. 4 12 0. 149 38 2 mil coat; shiny, smooth, soft,flexible.

Kovar 1 1, 000 5 +0. 3 8 0. 095 29 0.5 mil coat; shiny, smooth,moderately hard, brittle.

Monel 900 3 +1.15 12 0.600 77 1 mil coat; shiny, smooth, moderatelyflexible and hard.

B-Titanium, 13% 1, 000 5 1. 85 0. 231 15 0.5 mil coat; grey, smooth,flexible, soft;

vanadium, 11% Gd plate overlaid inner layer. chromium, 3% aluminum.

Zircouided 2 3 1, 000 15 +0.23 0.7 0. 040 14 0.2 mil outer coat; shiny,smooth, hard Niobium. over 1.5 mil soft inner layer; total coatingflexible.

Borided 1 Ti-Namel 1, 000 5 +0.33 4.0 0. 153 47 0.5 mil outer coat,shiny, smooth, very hard over 1.5 mil inner coat; total coatingflexible. 1

Scandided 1 4 nickel... 1, 000 60 O. 380. 08 0. 75 0. 864 88 1.5 milouter layer, shiny, smooth, very hard, over 1 mil inner layer,moderately hard; total coating flexible.

1 Gadolinium anode.

2 Shielded carbon anode.

3 The niobium was zirconided in accordance with the procedure set Itwill, of course, be apparent to those skilled in the art thatmodifications other than those set forth in the above examples can beemployed in the process of this invention without departing from thescope thereof.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A method of forming a metallide coating of a metal of the groupconsisting of yttrium, cerium, praseodymium, neodymium, promethium,Samarium, europium, gadolinium, terbium, dysprosium, holmium, erbiumthulium, ytterbium and lutetium on a base metal selected from the groupconsisting of (a) metal compositions having a melting point greater than900 C., at least 50 mole percent of which is at least one metal selectedfrom the group of metals whose atomic numbers are 4, 21, 22, to 29, 40,43 to 47, 72 and 75 to 79 and (b) a member of the class consisting ofberyllided, borided, and silicided, vanadium, chromium, niobium,molybdenum, tantalum and tungsten, which comprises (1) forming anelectric cell containing said base metal as the cathode, a carbon anodeor said coating metal as an anode and a fused salt electrolyteconsisting esentially of a member of the class consisting of lithiumfluoride, sodium fluoride, mixtures thereof and mixtures of the saidfluorides with barium fluoride or strontium fluoride, and containingfrom 0.01 to 40 mole percent of a fluoride of the coating metal, saidelectrolyte being maintained at a temperature of at least 900 C., butbelow the melting point of said metal composition in the substantialabsence of oxygen, (2) passing a current between said anode and saidcathode sufficient to establish a cathode current density of from 0.01to amperes per square decimetcr and a cell voltage below 3.0 volts, (3)interrupting the flow of electrical current after the desired thicknessof the metallide coating is formed and (4) immediately removing themetallided base metal composition from the fused salt electrolyte.

2. The method of claim 1 wherein the carbon anode is shielded with atightly woven metal screen, such as niobium, molybdenum, tantalum,tungstun and iron.

3. A method of forming a yttride or rare earthide coating on a basemetal selected from the class consisting of (a) metal compositionshaving a melting point greater than 900 C., at least 50 mole percent ofsaid metal composition being at least one of the metals selected fromthe class consisting of metals whose atomic numbers are 4, 21, 22, 25 to29, 40, 43 to 47, 72 and, 75 to 79, and (b) a member of the classconsisting of beryllided, borided and silicided vanadium, chromium,niobium, molybdenum, tantalum and tungsten, said method comprising (1)forming an electric cell containing said base metal as the cathode,joined through an external electrical circuit to a yttrium or rare earthmetal anode and a fused salt forth in my application Ser. No. 593,274,filed concurrently herewith.

4 The nickel was scandided in accordance with the procedure set forth inmy application Ser. No. 593,270, filed concurrently herewith.electrolyte which consists essentially of a member of the classconsisting of lithium fluoride, sodium fluoride, mixtures thereof andmixtures of said fluorides with strontium fluoride or barium fluorideand from"-0.l40 mole percent of yttrium fluoride or a rare earthfluoride, said electrolyte being maintained at a temperature of at least900 C., but below the melting point of said metal composition in thesubstantial absence of oxygen, (2) controlling the current flowing insaid electric cell so that the current density aof the cathode does notexceed 10 amperes/dm. during the formation of the metallide coating, and(3) interrupting the flow of electrical current after the desiredthickness of the yttride or rare earthide coating is formed on the metalcomposition.

4. The method of claim 3 wherein the fused salt electrolyte consistsessentially of lithium fluoride and the fluoride of the metal beingdeposited to form the metallided coating.

5. The method of claim 3 which is also conducted in the substantialabsence of carbon.

6. The method of claim 3 wherein the absence of oxygen is obtained byuse of an inert gas in the cell.

7. The method of claim 3 wherein the metal composition is nickel.

8. The method of claim 3 wherein the metal composition is cobalt.

9. The method of claim 3 wherein the metal composition is titanium.

10. The method of claim 3 wherein the metal composition is zirconium.

11. The product produced by the process of claim 1.

References Cited UNITED STATES PATENTS 2,828,251 3/1958 Sibert et al.204-39 3,024,175 3/1962 Cook 204-39 3,024,176 3/ 1962 Cook 204-39 Re.25,630 8/1964 Cook 204--39 3,232,853 2/1966 Cook 20439 FOREIGN PATENTS563,495 9/ 1958 Canada. 742,190 9/1966 Canada.

OTHER REFERENCES I. Electrochemical Society, vol. 112, No. 3, 1965, pp.266-272.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl.X.R. 204-39

